Basically the two major programs for music notation are Finale and Sibelius. Finale is the most widely used right now, sort of the Microsoft Office of music notation. However, its interface can be pretty weird sometimes. Sibelius is said to have a much better interface, and they generally are much more Mac-friendly (Finale didn't even have an OS X version until last year). But they have a smaller user base, and of course, neither program can save in the other's file format.
Best thing to do is probably to go to a university's music library where they will have both installed. Try them both, and see which you like better.